1. Field of the Invention
The present invention relates to a nitride semiconductor device using a Group III nitride semiconductor.
2. Description of Related Art
An HEMT (High Electron Mobility Transistor) is conventionally known as a low-noise amplifying device to be equipped in an antenna for receiving satellite broadcast, a car navigation system, or the like.
An HEMT includes, for example, a Si substrate, an undoped GaN (gallium nitride) layer as an electron transit layer laminated on the Si substrate, and an n-type AlGaN (aluminum gallium nitride) layer as an electron supply layer laminated on the undoped GaN layer. Near an interface between the undoped GaN layer and the n-type AlGaN layer, a two-dimensional electron gas is formed inside the undoped GaN layer due to piezoelectric polarization resulting from a heterojunction between the undoped GaN layer and the n-type AlGaN layer. On the n-type AlGaN layer, a gate electrode is formed so as to form a Schottky junction with the n-type AlGaN layer. Also, on the n-type AlGaN layer, a source electrode and a drain electrode are formed to oppose each other with the gate electrode being interposed between the source electrode and the drain electrode. The source electrode and the drain electrode form ohmic contact with the n-type AlGaN layer. The source electrode and the drain electrode are thus electrically connected to the two-dimensional electron gas through the n-type AlGaN layer.
The HEMT having the above structure has a so-called normally-on characteristic such that even when a voltage is not applied to the gate electrode, conduction is provided between the source electrode and the drain electrode through the two-dimensional electron gas as long as there is a potential difference between the source and the drain. Thus, to put the HEMT in an off state, a negative voltage must be applied to the gate electrode to pinch off the two-dimensional electron gas. Attention is drawn to an HEMT of so-called normally-off characteristic such that the source-drain interval is put in an interrupted state even when a voltage is not applied to the gate electrode.
As an HEMT with a normally-off characteristic, an HEMT has been proposed that a portion of the electron supply layer at which the gate electrode is to be formed is formed thinly (for example, the corresponding location is made 20 nm in layer thickness) by recess etching. With this HEMT, even in the state in which a voltage is not applied to the gate electrode, the two-dimensional electron gas is pinched off by a depletion layer below the gate electrode (a depletion layer extending from the Schottky junction). A normally-off operation, in which the interval between the source electrode and the drain electrode is nonconductive is thus realized.
However, with the arrangement where the portion of the electron supply layer at which the gate electrode is formed is made thin in layer thickness, it is difficult to control a gate threshold voltage to a desired voltage value. For example, there are cases where the gate threshold voltage cannot be controlled to a voltage value of +1V or more.